Systems for manufacturing commercial products of rare earth magnet include a single part system wherein a part of substantially the same shape as the product is produced at the stage of press molding, and a multiple part system wherein once a large block is molded, it is divided into a plurality of parts by machining. These systems are schematically illustrated in FIG. 1. FIG. 1A illustrates the single part system including press molding, sintering or heat treating, and finishing steps. A molded part P101, a sintered or heat treated part P102, and a finished part (or product) P103 are substantially identical in shape and size. Insofar as normal sintering is performed, a sintered part of near net shape is obtained, and the load of the finishing step is relatively low. However, when it is desired to manufacture parts of small size or parts having a reduced thickness in magnetization direction, the sequence of press molding and sintering is difficult to form sintered parts of normal shape, leading to a lowering of manufacturing yield, and at worst, such parts cannot be formed.
In contrast, the multiple part system illustrated in FIG. 1B eliminates the above-mentioned problems and allows press molding and sintering or heat treating steps to be performed with high productivity and versatility. It now becomes the mainstream of rare earth magnet manufacture. In the multiple part system, a molded block P101 and a sintered or heat treated block P102 are substantially identical in shape and size, but the subsequent finishing step requires cutting. It is the key for manufacture of finished parts P103 how to cutoff machine the block in the most efficient and least wasteful manner.
Well-known methods for cutoff machining of rare earth magnet blocks include a wire cutting method using a wire having abrasive grains bonded to the surface thereof, an outer- and inner-diameter cutting methods using outer- and inner-diameter blades.
Tools for cutting rare earth magnet blocks include two types, a diamond grinding wheel inner-diameter (ID) blade having diamond grits bonded to an inner periphery of a thin doughnut-shaped disk, and a diamond grinding wheel outer-diameter (OD) blade having diamond grits bonded to an outer periphery of a thin disk as a core. Nowadays the cutoff machining technology using OD blades becomes the mainstream, especially from the aspect of productivity. The machining technology using ID blades is low in productivity because of a single blade cutting mode. In the case of OD blade, multiple cutting is possible. FIG. 2 illustrates an exemplary multiple blade assembly 5 comprising a plurality of cutoff abrasive blades 51 coaxially mounted on a rotating shaft 52 alternately with spacers (not shown), each blade 51 comprising a core 51b in the form of a thin doughnut disk and an abrasive grain layer 51a on an outer peripheral rim of the core 51b. This multiple blade assembly 5 is capable of multiple cutoff machining, that is, to machine a block into a multiplicity of parts at a time.
When a rare earth magnet block is machined by a multiple blade assembly, the magnet block is generally secured to a carbon-based support by bonding with wax or a similar adhesive which can be removed after cutting. The bonding with wax is achieved by heating the carbon-based support and the magnet block, applying molten wax between the support and the magnet block, and cooling for solidification. In this state, the magnet block is cut into pieces. The cutting operation is followed by heating to melt the wax, allowing the magnet pieces to be removed from the support. Since wax is kept attached to the magnet pieces at this point, the wax must be removed using a solvent or the like.
The adhesive way of securing a magnet block with wax involves concomitant steps of heat bonding, heat stripping and cleaning in addition to the cutting step, rendering the process very cumbersome. As a result, the cost of the cutting process is increased. One solution to this problem is a means for holding a magnet block without a need for wax, specifically a holding jig which is comb-shaped so as to allow passage of cutting blades during cutting.
For example, JP-A H06-304833 and JP-A 2001-212730 disclose a mechanism comprising a jig segment pivotally mounted for holding a workpiece on a support. Since the shape and size of a workpiece which can be held by the jig are limited, a jig must be separately prepared for a particular shape of workpiece. In some cases, an elastic member is disposed between the jig and the workpiece. Under the stress induced during the holding, the elastic member can be deformed in a complex way. It is then impossible to hold the magnet block at the same posture before and after the cutting operation. If magnet pieces are inclined immediately after cutting, they may contact with the cutting blades. As a result, the magnet pieces may be abraded and degraded in dimensional accuracy, and the cutting blades be damaged.
JP-A 2007-044806 proposes an apparatus for clamping a magnet block by resin-based jigs. The magnet block is disposed between U-shape recessed jigs, and the magnet block is held in place by deformation of the jigs. Since the force applied to the magnet block by cutting blades during the cutting operation acts to push downward the lower fingers of the U-shape recessed jigs, the U-shape is outward expanded, failing to keep the clamped state until the end.
In the jigs disclosed in JP-A 2007-044806 and JP-A 2000-280160, the cutting direction is set vertical. The cutting distance is limited to the distance of downward movement of a cutting blade assembly. This inhibits an efficient arrangement wherein a plurality of workpieces are arranged in tandem in the cutting direction.
JP-A 2006-068998 discloses a jig comprising a resinous support and a resinous frame for holding a magnet block on the support. It is difficult to uniformly distribute the holding force of the frame over the entire magnet block and to hold discrete magnet pieces after cutting. As the thickness of cut magnet pieces decreases, the resinous frame becomes thinner, failing to maintain strength. Since the magnet block is loaded to and unloaded from the jig by screw engagement, the loading/unloading operation is cumbersome.
Most of the foregoing patent documents relate to a mechanism for clamping a workpiece by a comb-shaped jig. As discussed for the respective documents, they have problems such as the limited shape of a magnet block and cumbersome loading/unloading operation. In fact, these mechanisms are difficult to hold a workpiece or magnet block in place until the completion of cutting. It is likely that immediately after cutting, magnet pieces move sideways and come in contact with the rotating cutting blades which are being retracted at the end of cutting. Then the magnet pieces may be abraded, resulting in dimensional degradation, and the interference between magnet pieces and cutting blades can cause magnet piece fissure and/or cutting blade damage.